The patch titled
Subject: mm/memcg: protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.
has been added to the -mm tree. Its filename is
mm-memcg-protect-per-cpu-counter-by-disabling-preemption-on-preempt_rt-where-needed.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-memcg-protect-per-cpu-counter-by-disabling-preemption-on-preempt_rt-where-needed.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-memcg-protect-per-cpu-counter-by-disabling-preemption-on-preempt_rt-where-needed.patch
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------------------------------------------------------
From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Subject: mm/memcg: protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.
The per-CPU counter are modified with the non-atomic modifier. The
consistency is ensured by disabling interrupts for the update. On non
PREEMPT_RT configuration this works because acquiring a spinlock_t typed
lock with the _irq() suffix disables interrupts. On PREEMPT_RT
configurations the RMW operation can be interrupted.
Another problem is that mem_cgroup_swapout() expects to be invoked with
disabled interrupts because the caller has to acquire a spinlock_t which
is acquired with disabled interrupts. Since spinlock_t never disables
interrupts on PREEMPT_RT the interrupts are never disabled at this point.
The code is never called from in_irq() context on PREEMPT_RT therefore
disabling preemption during the update is sufficient on PREEMPT_RT. The
sections which explicitly disable interrupts can remain on PREEMPT_RT
because the sections remain short and they don't involve sleeping locks
(memcg_check_events() is doing nothing on PREEMPT_RT).
Disable preemption during update of the per-CPU variables which do not
explicitly disable interrupts.
Link: https://lkml.kernel.org/r/20220221182540.380526-4-bigeasy@xxxxxxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Acked-by: Roman Gushchin <guro@xxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: kernel test robot <oliver.sang@xxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Michal Koutný <mkoutny@xxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 49 +++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 48 insertions(+), 1 deletion(-)
--- a/mm/memcontrol.c~mm-memcg-protect-per-cpu-counter-by-disabling-preemption-on-preempt_rt-where-needed
+++ a/mm/memcontrol.c
@@ -629,6 +629,35 @@ static DEFINE_SPINLOCK(stats_flush_lock)
static DEFINE_PER_CPU(unsigned int, stats_updates);
static atomic_t stats_flush_threshold = ATOMIC_INIT(0);
+/*
+ * Accessors to ensure that preemption is disabled on PREEMPT_RT because it can
+ * not rely on this as part of an acquired spinlock_t lock. These functions are
+ * never used in hardirq context on PREEMPT_RT and therefore disabling preemtion
+ * is sufficient.
+ */
+static void memcg_stats_lock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+ preempt_disable();
+#else
+ VM_BUG_ON(!irqs_disabled());
+#endif
+}
+
+static void __memcg_stats_lock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+ preempt_disable();
+#endif
+}
+
+static void memcg_stats_unlock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+ preempt_enable();
+#endif
+}
+
static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
{
unsigned int x;
@@ -705,6 +734,20 @@ void __mod_memcg_lruvec_state(struct lru
pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
memcg = pn->memcg;
+ /*
+ * The caller from rmap relay on disabled preemption becase they never
+ * update their counter from in-interrupt context. For these two
+ * counters we check that the update is never performed from an
+ * interrupt context while other caller need to have disabled interrupt.
+ */
+ __memcg_stats_lock();
+ if (IS_ENABLED(CONFIG_DEBUG_VM)) {
+ if (idx == NR_ANON_MAPPED || idx == NR_FILE_MAPPED)
+ WARN_ON_ONCE(!in_task());
+ else
+ WARN_ON_ONCE(!irqs_disabled());
+ }
+
/* Update memcg */
__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
@@ -712,6 +755,7 @@ void __mod_memcg_lruvec_state(struct lru
__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
memcg_rstat_updated(memcg, val);
+ memcg_stats_unlock();
}
/**
@@ -794,8 +838,10 @@ void __count_memcg_events(struct mem_cgr
if (mem_cgroup_disabled())
return;
+ memcg_stats_lock();
__this_cpu_add(memcg->vmstats_percpu->events[idx], count);
memcg_rstat_updated(memcg, count);
+ memcg_stats_unlock();
}
static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
@@ -7154,8 +7200,9 @@ void mem_cgroup_swapout(struct page *pag
* important here to have the interrupts disabled because it is the
* only synchronisation we have for updating the per-CPU variables.
*/
- VM_BUG_ON(!irqs_disabled());
+ memcg_stats_lock();
mem_cgroup_charge_statistics(memcg, -nr_entries);
+ memcg_stats_unlock();
memcg_check_events(memcg, page_to_nid(page));
css_put(&memcg->css);
_
Patches currently in -mm which might be from bigeasy@xxxxxxxxxxxxx are
mm-memcg-disable-threshold-event-handlers-on-preempt_rt.patch
mm-memcg-protect-per-cpu-counter-by-disabling-preemption-on-preempt_rt-where-needed.patch
mm-memcg-protect-memcg_stock-with-a-local_lock_t.patch
mm-memcg-disable-migration-instead-of-preemption-in-drain_all_stock.patch
